1. Field of the Invention
The present invention relates to a radiation detector assembly and also to a sample analyzer.
2. Description of the Related Art
Radiation detector assemblies are apparatus for detecting radiations (such as X-rays and gamma rays (γ-rays)) and constructing radiation spectra. Known radiation detector assemblies of this type include energy-dispersive X-ray spectrometers (EDS) and wavelength-dispersive X-ray spectrometers (WDS).
In an energy-dispersive X-ray spectrometer, a distribution of pulse heights is created by a multichannel analyzer (MCA). Since this is a distribution of pulse heights at this moment, an X-ray spectrum is not yet completed. Accordingly, the pulse heights are converted into X-ray energies, thus producing an X-ray spectrum. Generally, the following formula (1) is used in converting pulse heights (ch) into X-ray energies (E):E=gain×ch+offset  (1)
In the energy-dispersive detector, an energy calibration is done, for example, by making a measurement on a reference sample containing a known element and used for energy calibration and by finding the gain and offset included in Eq. (1) above.
For example, in an X-ray fluorescent analyzer using an energy-dispersive detector, energy positions may deviate due to aging of the detector and signal processing circuitry in the stage following the detector. That is, positions taken along the horizontal axis of the X-ray fluorescent spectrum may deviate. If energy position deviations increase to some extent, and if a spectral line of a certain element is present, the difference between the energy corresponding to the spectral line and the theoretical energy corresponding to the element is increased. This creates the danger that the spectral line will not be identified as corresponding to the element or that the spectral line might be misidentified as corresponding to other element, i.e., an incorrect elemental assignment is made. Accordingly, in order to correct for deviation of energy position as described above, an energy calibration is done.
For example, JP-A-10-48161 discloses an X-ray fluorescent analyzer having a shutter to which a reference sample for energy calibration is directly attached. Thus, the reference sample is kept in the instrument. This eliminates the labor to exchange the reference sample. Hence, this analyzer permits an energy calibration to be done quickly and easily.
In the past, the analyzer himself or herself has made a decision as to whether an energy calibration routine as described above is needed. In particular, the use time and number of uses of the instrument are managed. It is determined that an energy calibration is needed whenever a given operating time has passed or a given number of uses are reached. Then, an energy calibration using a sample for energy calibration is carried out.
With this method, however, it is not always possible to appropriately determine whether the condition needs an energy calibration. Therefore, an energy calibration may be carried out in spite of the condition in which no calibration is necessary.
Furthermore, if the frequency of energy calibration operations is lowered in an attempt to avoid unwanted energy calibration operations, then there is the danger that the energy position deviation might exceed the tolerable range, for example, during continuous measurement on plural samples. In this case, it is not obvious when the energy position deviation exceeded the tolerable range. Consequently, all the results of measurements already obtained are discarded because they are regarded as unreliable. Alternatively, it may be necessary to perform a work for evaluating whether each individual measurement result is correct or not.